The present invention relates to an electrophotographic image forming apparatus, and more particularly to an image forming apparatus which is capable of maintaining appropriate relative positions between an image carrier and a developing unit which uses toner to develop an electrostatic latent image formed on the image carrier.
Image forming apparatuses, such as electrophotographic copying machines and printers, include apparatuses for visualizing an electrostatic latent image formed on an image carrier (hereinafter typified by a "photosensitive member"), such as a photosensitive drum, by a developing unit thereof as a toner image so as to directly transfer the toner image on a recording medium. Apparatuses of another type are known each of which incorporates an intermediate transfer member which comprises a drum or an endless-film-type belt member and to which the toner image is temporarily and primarily transferred. The primarily-transferred toner image is secondarily transferred to the surface of a recording medium.
FIG. 1 is a diagram showing an example of the image forming apparatus. Referring to the drawing, a charging roller 8, an exposing unit 5, a developing unit assembly 10, a primary transfer roller 2 and a cleaner unit 11 are disposed around a photosensitive member 1 which is rotated by a motor (not shown) in a direction indicated by an arrow B. The developing unit assembly 10 incorporates four developing units 10Y, 10M, 10C and 10K for performing a full-color developing process. Each of the developing units 10Y, 10M, 10C and 10K corresponding to toners in yellow (Y), magenta (M), cyan (C) and black (K) for developing a latent image on the photosensitive member 1 has an interchangeable toner cartridge, a developing roller for imparting a developing bias, a toner supply unit for supplying the toner to the developing roller and a moving unit (each of which is not shown). When latent images corresponding to the foregoing colors are developed, a motor (not shown) is rotated to sequentially rotate the developing unit assembly 10 by an angular degree of 90.degree. in a direction opposite to the direction B, that is, in a direction indicated by an arrow A. Thus, positioning is performed to cause the developing roller in a required color to be brought into contact with the photosensitive member 1.
Toner images in the foregoing colors are sequentially formed on the photosensitive member 1, the toner images being sequentially transferred to the surface of an intermediate transfer belt (hereinafter simply called a "belt") 3 by the primary transfer roller 2 so that toner images in the four colors are superimposed. The belt 3 is arranged among rollers 12, 13, 14 and 15. The roller 12 serves as a drive roller, the roller 13 serves as a tension roller for imparting a tension to the belt 3 and the roller 14 serves as a backup roller for the secondary transfer roller 4. A cleaner 16 for removing toner left on the belt 3 is disposed at a position opposite to the roller 15 across the belt 3.
Recording paper drawn from the recording sheet cassette 17 or 18 by the drawing roller 19 or 20 is moved to a contact portion (or a secondary transferring portion) between the secondary transfer roller 4 and the belt 3 by roller pairs 21, 22 and 23. Urging of the secondary transfer roller 4 toward the roller 14 is suspended, and thus the secondary transfer roller 4 is retracted to a downward position until a secondary transferring operation is performed. When the secondary transference is performed, the secondary transfer roller 4 is brought into contact with the belt 3. Thus, a toner image formed on the belt 3 is, in the contact portion, transferred to the surface of the recording paper, and then fixed with heat in a fixing unit 24. Then, the recording paper is discharged to a tray 25 or a tray 26 (disposed on the upper surface of the apparatus).
A reflecting-type optical sensor 6 is disposed opposite to the belt 3 so as to detect a light beam reflected by a marking (not shown) in the form of aluminum foil disposed on the surface of the belt 3. An output of the reflecting-type optical sensor 6 is used as a reference signal for controlling timing at which the exposing unit 5 forms an image and timing at which a toner image is transferred.
When the image forming apparatus is operated, voltage is applied to the charging roller 8 to negatively charge the surface of the photosensitive member 1 to a predetermined potential for the charged portion. An image portion on the charged photosensitive member 1 has a predetermined potential for the exposed portion by an exposing operation performed by the exposing unit 5. Thus, an electrostatic latent image is formed. The developing roller of the developing unit 10Y and the like is previously applied with a developing bias previously determined for each color. Thus, the latent image is developed with the toner at the time of passage through the position of the developing roller. The toner image is transferred to the belt 3 by the primary transfer roller 2, and then transferred to the recording paper by the secondary transfer roller 4 so as to be supplied to the fixing unit 24. When a full-color printing operation is performed, toners in the four colors are superimposed on the belt 3 so as to be transferred to the recording paper.
The developing roller of the above-mentioned image forming apparatus is a developing roller having a plurality of magnetic poles to utilize magnetic force to absorb a carrier which forms the developer together with the toner so that a magnetic brush is formed which supplies the toner absorbed to the carrier to the photosensitive member. magnetic lines of forces corresponding to the number of the magnetic poles are generated around the developing roller. The intensity of the magnetic force varies depending on the circumferential position on the developing roller. Therefore, the quantity of toner which is adsorbed by the developing roller varies depending on the position.
On the other hand, the gear of the drive motor for rotating the developing unit assembly and the gear of the developing unit assembly arranged to be engaged with the gear of the drive motor possess a backlash. Therefore, even if the drive motor is stopped at the reference position to fix each developing roller to a predetermined position with respect to the photosensitive member, the stopping position of the developing roller cannot be stabilized because of the backlash. As a result, the relative positions between the photosensitive member and the developing roller cannot be stabilized.
If the relative positions between the photosensitive member and the developing roller cannot be stabilized when the development is performed, the quantity of toner which is supplied to the photosensitive member varies depending on the relationship between the position of the developing roller and the quantity of the toner. As a result, there arises a problem in that the density of a formed image becomes unstable. FIG. 2 is a graph showing the examined relationship between the stopping positions of the developing roller and the densities of images. The stopping position is indicated with the distance realized on the surface of the photosensitive member from a straight line connecting the center of the developing unit assembly and that of the photosensitive member to each other to the point of contact between the developing roller and the photosensitive member. The distance in the direction of rotation of the photosensitive member is the positive direction, while the distance in the opposite direction is the negative direction. As shown in the graph, the density of the image is raised when the stopping position of the developing roller is shifted in the positive direction.
FIG. 3 is a perspective view showing the structure of a drive portion of the developing unit assembly 10. The rotary developing unit assembly 10 has a helical gear 78 secured coaxially with a main shaft 68 of the developing unit assembly 10. A helical gear 71 secured to a output shaft 70a of a step motor 70 is engaged with the helical gear 78 as shown in this figure. When a required developing unit is moved to a position opposite to the photosensitive member 1, a predetermined number of pulses are supplied to the step motor 70 so that the developing unit assembly 10 is rotated to a predetermined rotational position.
FIG. 4 is a diagram showing a method by which the developing unit assembly 10 is supported by a device frame 90. The developing unit assembly 10 has the main shaft 68, the both ends of which are rotatively supported by the device frame 90. The position of the developing unit assembly 10 in, for example, the axial direction, is fixed by engaging the stop ring 91 to the main shaft 68.
FIGS. 5 and 6 are schematic plan views each showing a state of engagement between the helical gear 78 of the developing unit assembly 10 and the helical gear 71 of the step motor 70. When the step motor 70 has been rotated to rotate the developing unit assembly 10, the helical gears 71 and 78 are operated as shown in FIG. 5 such that helical teeth 71a of the helical gear 71 (the step motor) push helical teeth 78a of the helical gear 78 (the developing unit assembly) in a direction of rotation.
The rotation of the step motor 70 is interrupted after the developing unit assembly 10 has been moved to the predetermined rotational position. Since inertia force is exerted on the developing unit assembly 10 in the direction of rotation, the developing unit assembly 10 cannot immediately be stopped. Therefore, when the step motor 70 is stopped, the helical teeth 78a of the helical gear 78 push the helical teeth 71a of the helical gear 71 by dint of the inertia force, as shown in FIG. 6.
At this time, inertial force W is exerted on the helical teeth 78a of the helical gear 78 in the direction of the rotation. Since the surface of contact between the helical teeth 71a and 78a is a smooth surface, vertical resistance N is exerted from the contact surface to the helical teeth 78a. Therefore, resultant force F is exerted on the helical gear 78 in parallel with the contact surface toward a direction shown in FIG. 6.
As described above, deviation of the developing unit assembly 10 in the axial direction is limited by the stop ring 91 engaged with the main shaft 68. However, the amount of deviation cannot completely be made to be zero because of existence of clearances and dimension tolerances. Therefore, when the resultant force F is produced, the helical gear 78 is displaced in parallel with the contact surface toward a direction indicated with an arrow Z shown in FIG. 3. Assuming that the distance of movement of the main shaft 68 along the contact surface is L and the helix angle of the helical teeth is .theta., the main shaft 68 is advanced excessively by L.cndot.sin .theta. in the direction of the rotation. Therefore, the contact position of the developing unit with respect to the photosensitive member 1 is undesirably deviated by L.cndot.sin .theta.. Thus, there arises a problem in that irregularity in development and change in the density occur.
To solve the above-mentioned problem, the main shaft 68 of the developing unit assembly 10 must accurately be positioned with respect to the device frame 90. When the accurate positioning is performed, an adjustment operation and the like are required after the apparatus has been assembled. Thus, there arises a problem in that the manufacturing process is complicated.